5 Steps to a 5 AP Chemistry 2019

164 ❯ STEP 4. Review the Knowledge You Need to Score High

❯ Answer and Explanation

(a) Silicon tetrafluoride is the only one of the three compounds that is not polar.

Si

SiF 4 : four bonding

pairs and no

lone pairs

Tetrahedral

F F

F

F

Xe

XeF 4 : four bonding

pairs and two

lone pairs.

Square planar

F F

F

F

S

SF 4 : four bonding

pairs and one

lone pair.

Seesaw

F F

F

F

You get 1 point if you correctly predict only SiF 4 to be nonpolar. You get 1 additional

point for each correct Lewis structure.

(b) The order is KrF 2 < SeF 2 < SnF 2.

F Kr F

FSe

F Sn 2

2+ −

F

KrF 2 : two bonding

pairs and three

nonbonding pairs

SeF 2 : two bonding

pairs and two

nonbonding pairs

SnF 2 : ionic

The Lewis structure indicates that KrF 2 is nonpolar. Thus, it has only very weak

London dispersion forces between the molecules. SeF 2 is polar, and the molecules are

attracted by dipole–dipole attractions, which are stronger than London. SnF 2 has the

highest melting point because of the presence of strong ionic bonds.

You get 1 point for the order and 1 point for the discussion.

(c) It is possible to draw the following resonance structures for the oxalate ion. The pres-

ence of resonance equalizes the bonds.

C

O

O^2 −

C

O

O

C

O

O^2 −

C

O

O O

O

C

O O

2 −

C

O

O

C

2 −

C

O

O

You get 1 point for any correct Lewis structure for C 2 O 42 - and 1 point for showing

or discussing resonance.